The invention relates to an electronic circuit with a logarithmic detector and to an apparatus having such detector, such as a digital cellular telephone.
A logarithmic detector provides an output signal that represents the logarithm of an input signal. Such a circuit is used in equipment that processes an input signal having a wide dynamic range, such as cellular radios, digital cellular telephones, spectrum analyzers, cordless telephones and radar equipment. The logarithmic detector typically functions as a received signal strength indicator (RSSI). In cellular radio, this function is necessary for continuously monitoring the received signal strength by the radio's microcontroller. The RSSI function requires a dc output voltage which is proportional to the logarithm of the input signal level.
Typically, such a detector is composed of a cascade of signal amplifiers. For the sake of clarity, it is assumed that the amplifiers have identical linear-amplification ranges and identical amplification "A". The first amplifier of the cascade receives an input signal "v" that is successively amplified by the subsequent amplifiers in the cascade. The repetitive amplification continues until a particular amplifier runs out of its linear range and into its limiting range. In its limiting range, each amplifier furnishes a clipped version of its amplified input signal. The n-th amplifier along the cascade runs into its limiting range when its output signal A.sup.n v exceeds the limit of the linear amplification range. Consequently, the input signal values at which the respective amplifiers leave their linear amplification ranges are proportional to A.sup.-n. Summing the output signals of all amplifiers approximates a linear function of the logarithm of the input signal.
In practice, the amplifiers are signal voltage amplifiers. The signals-supplied by the cascade have to be rectified in order to generate a dc output quantity representative of the signal's strength. Generally, an interstage filter is inserted into the cascade for reducing noise and preventing regeneration. Such a filter, however, causes undesired losses in amplitude. The output voltages therefore usually are rescaled for loss-compensation before being processed further.
In prior art integrated circuit NE605, manufactured by Signetics Co. (Philips Semiconductors), the output voltages of the amplifiers are routed to a rectifying element via a resistor ladder formed of stacked pairs of resistors. The amplifiers have complementary outputs connected to nodes between particular pairs of resistors. The voltage division attained in this way takes care of the rescaling of the signal. The rescaling, however, also affects the various dc levels of the voltages at the ladder nodes. These rescaled dc levels are to be removed from the rectifier output signal in a subsequent circuit.
The supply voltage required for operating the prior art circuit therefore is determined, among other things, by the resistor ladder. Further, removing the dc levels from the rectified signal requires additional circuitry. As a result, the detector has to be redesigned for each particular combination of rescaling resistors. Also, since various resistor values are used in a ladder, the accuracy of the resistor values should be high in order to reduce the accumulation of signal distortion in the ladder and beyond, due to temperature influences and process parameter spread dependencies.